Correlation between the induction of an immediate early gene, zif/268, and long-term potentiation in the dentate gyrus.

Expression of the immediate early gene zif/268 (also termed NGFI-A, Krox 24, TIS8 and Egr-1) was investigated in awake rats following various long-term potentiation (LTP) induction protocols.zif/268 mRNA (Northern blots) and protein (immunohistochemistry) levels sharply increased following LTP, and followed a time course characteristic of other immediate early genes. When measured across 3 tetanization protocols known to produce differing degrees of LTP persistence,zif/268 induction was found to be more highly correlated with LTP duration than with the magnitude of initial LTP. These data support the hypothesis that the immediate early gene zif/268 plays a role as a third messenger in the cascade of cellular and nuclear events that govern the persistence of LTP.     

Correlation between the induction of an immediate early gene,zif/268, and long-term potentiation in the dentate gyrus

Expression of the immediate early gene zif/268 (also termed NGFI-A, Krox 24, TIS8 and Egr-1) was investigated in awake rats following various long-term potentiation (LTP) induction protocols.zif/268 mRNA (Northern blots) and protein (immunohistochemistry) levels sharply increased following LTP, and followed a time course characteristic of other immediate early genes. When measured across 3 tetanization protocols known to produce differing degrees of LTP persistence,zif/268 induction was found to be more highly correlated with LTP duration than with the magnitude of initial LTP. These data support the hypothesis that the immediate early gene zif/268 plays a role as a third messenger in the cascade of cellular and nuclear events that govern the persistence of LTP.     

Noradrenergic activation of immediate early genes in rat cerebral cortex.

Previous studies have shown that stimulation of adrenergic receptors in the brain increases the expression of the immediate early gene (IEG), c-fos, in vivo (Mol. Brain Res., 6(1989) 39-45). The present study was undertaken to determine whether this also holds for other IEGs which have been shown to be activated in brain cell culture by adrenergic agonists. Both yohimbine injection and stressful stimulation, two treatments causing brain norepinephrine (NE) release, were found to cause a parallel, transient activation of at least 5 IEGs (c-fos, nur77, tis-7, zif-268 and tis-21) in the rat cortex. Genes that are not immediate early (beta-actin, NGF and HSP70) were found not to be affected in the interval used (6 h). The responses were mediated predominantly by beta-adrenoceptors with some contribution from alpha 1 receptors. The parallel activation of multiple genes by noradrenergic receptors may enable the coding of different biochemical responses to the activation of different receptors.     

The relationship between extracellular calcium concentrations and the induction of hippocampal long-term potentiation

The effect of varying calcium and magnesium concentrations was observed on the development of long-term potentiation (LTP) in the CA1 region of the rat hippocampus maintained in vitro. Treatments which would be expected to lower the depolarization-induced influx of calcium into neuronal elements (i.e. lowering Ca2+ or raising Mg2+ in the perfusion medium) selectively antagonized LTP, while synaptic transmission, paired-pulse and frequency facilitation, as well as short-term (or post-tetanic) potentiation were all maintained. This suggests that LTP is unrelated to these short-term forms of plasticity, but is either a calcium-dependent process itself, or is dependent upon some other calcium-related phenomenon (such as transmitter release).     

Triazolam suppresses the induction of hippocampal long-term potentiation

Benzodiazepines are sedative hypnotics that produce marked anterograde amnesia in humans. These pharmacological properties are thought to result from the potentiation of GABA-A receptor function and subsequent attenuation of long-term potentiation (LTP), however many reports have suggested this is not the case for triazolam. Using electrophysiological recordings in a cell line expressing recombinant GABA-A receptors, we confirm that triazolam is an efficacious positive allosteric modulator of GABA-A receptors. Triazolam also slowed the decay of spontaneous inhibitory synaptic currents, reduced the amplitude of fEPSPs elicited during a theta burst and reduced the magnitude of LTP in hippocampal CA1 neurones in vitro. These data show that triazolam modifies LTP induction consistent with an enhancement of GABA-A receptor function via activation of the allosteric benzodiazepine-site.     

Elevation of extracellular potassium facilitates the induction of hippocampal long-term potentiation.

Hippocampal long-term potentiation (LTP) is widely believed to be a cellular substrate for learning and memory. A likely physiological stimulus for initiating LTP is repetitive neuronal activity, which also results in K+ accumulation extracellularly. Therefore, the involvement of elevated extracellular K+ concentrations in the induction of LTP of the stratum radiatum-CA1 neuronal synapse was investigated in the hippocampal slice preparation. Increasing the K+ content in extracellular perfusing medium from 3.1 to 15 mM resulted in facilitation of LTP induction in weak excitatory postsynaptic potentials (EPSPs). Since changes that occur to generate LTP are thought to be localized to synaptic regions, it would be relevant to selectively increase synaptic K+ levels. To this end, the following experiments were conducted: i) baclofen, a GABAB receptor agonist which, in addition to having a disinhibitory presynaptic action, activates a K+ conductance in CA1 neuronal dendrites, was applied to the slice; ii) K+ was directly applied by iontophoresis. At a concentration of 5 microM baclofen, as well as with K+ iontophoresis (200-300 nA), LTP of weak EPSPs was facilitated. The present data suggest that an increase in synaptic K+ levels can fulfill the condition of cooperativity for LTP induction, raising the possibility that an elevation of this monovalent ion plays a physiological role in triggering LTP.     

Lithium attenuates stress-induced impairment of long-term potentiation induction

Stress impairs the induction of long-term potentiation in the hippocampus as well as hippocampus-dependent memory. Lithium, a classical mood stabilizer, is known to have beneficial effects on stress-induced impairment of spatial memory. In the present study, we investigated lithium effects on the impairment of long-term potentiation induction after exposure to acute immobilization stress. As previously reported, immobilization stress impaired long-term potentiation induction in the CA1 region of rat hippocampal slices. Treating the slices with 0.6 or 1 mM lithium attenuated impaired long-term potentiation induction in stressed animals. Lithium was without effect on long-term potentiation induction in unstressed animals or baseline synaptic responses in unstressed or stressed animals. These results demonstrate a protective effect of lithium against stress-induced impairment of long-term potentiation induction.     

Differential effects of ketamine and MK-801 on the induction of long-term potentiation.

Ketamine and MK-801 are phencyclidine (PCP)-like noncompetitive antagonists of the N-methyl-D-aspartate (NMDA) receptor that produce a use-dependent blockade of the NMDA receptor-coupled channel. Recent studies have suggested that the binding properties of these drugs to the NMDA receptor in-vitro are different. In the present study, the effects of ketamine and MK-801 on the induction of long-term potentiation (LTP) were compared at perforant path--granule cell synapses in anaesthetized rats. LTP was observed in animals treated with either saline or MK-801, but not in those treated with ketamine. These results reveal that ketamine and MK-801 differentially modulate the induction of LTP, and we propose that this differential modulation may be related to the different binding properties of the drugs.     

Activation of immediate early genes after acute stress.

We used in-situ hybridization to study the effect of acute stress on induction of the immediate early genes (IEGs), c-fos and zif/268, in the rat brain. After one hour of restraint plus intermittent tail shock, messenger RNA (mRNA) levels for both genes were significantly increased bilaterally in the neocortex, particularly in layers IV, V and VI, and in the CA1 region of the hippocampus. This regionally-specific response suggests that IEGs may have a role in the mediation of acute stress responses in the central nervous system.     

Post-activation potentiation in the neocortex. IV. Multiple sessions required for induction of long-term potentiation in the chronic preparation

The neocortex in chronically prepared rats is very resistant to the induction of long-term potentiation (LTP). In the first of two experiments described in this paper, we tried unsuccessfully to induce neocortical LTP within one session by coactivating basal forebrain cholinergic and cortical inputs to our neocortical recording site. In the second experiment, we tested a new procedure which involved the application of repeated conditioning sessions over several days. This procedure was suggested by our finding that kindling-induced potentiation (KIP) of cortical field potentials could be reliably triggered but was slow to develop. We administered 30 high frequency trains per day to the corpus callosum for 25 days. LTP in callosal-neocortical field potentials became clear after about 5 days of stimulation and reached asymptotic levels by about 15 days. After the termination of treatment, LTP persisted for at least 4 weeks, the duration of our post-stimulation test period. As in previous experiments on kindling-induced potentiation, the potentiation effects were clear in both early population spike components and in a late (probably disynaptic) component. The monosynaptic EPSP component was often depressed, but this may have been due to competing field currents generated by the enhanced population spike activity. We discuss these results in the context of theories emphasizing slower but more permanent memory storage in neocortex compared to the hippocampus.     

Role of taurine uptake on the induction of long-term synaptic potentiation

Taurine application in the CA1 area of rat hippocampal slices induces a long-lasting potentiation of excitatory synaptic transmission that has some mechanistic similitude with the late phase of long-term potentiation (L-LTP). Previous indirect evidence such as temperature and sodium dependence indicated that taurine uptake is one of the primary steps leading to the taurine-induced synaptic potentiation. We show that taurine-induced potentiation is not related to the intracellular accumulation of taurine and is not impaired by 2-guanidinoethanesulphonic acid, a taurine transport inhibitor that is a substrate of taurine transporter. We have found that taurine uptake in hippocampal synaptosomes was inhibited by SKF 89976A, a GABA uptake blocker that is not transportable by GABA transporters. SKF 89976A prevents the induction of synaptic potentiation by taurine application. This effect is neither mimicked by nipecotic acid, a broad inhibitor of GABA transporters that does not affect taurine uptake, nor by NO-711, a specific and potent inhibitor of GABA transporter GAT-1. In addition, L-LTP induced by trains of high-frequency stimulation is also inhibited by SKF 89976A, and taurine, at a concentration that does not change basal synaptic transmission, overcomes such inhibition. We conclude that taurine induces synaptic potentiation through the activation of a system transporting taurine and that taurine uptake is required for the induction of synaptic plasticity phenomena such as L-LTP.     

Systematic expression of immediate early genes and intensive astrocyte activation induced by intrastriatal ferrous iron injection

The potential role(s) of transitional metals such as iron have been implicated in neurodegeneration through biochemical processes, particularly oxidative stress. We injected ferrous chloride (FeCl2) and ferric chloride (FeCl3) into the striatonigral system of Sprague-Dawley rats to investigate the biological and toxic effects of ferrous iron in the central nervous system. When FeCl2 was injected into the ventral midbrain, rats showed a characteristic behavior which indicated ipsilateral dopaminergic hyperactivity. FeCl2 injection into the striatum induced a dose-dependent damage, the activation of astrocytes and recruitment of macrophage/microglia at the injected site. Interestingly, the activation of astrocytes was also observed in the anatomically remote areas such as the ipsilateral subthalamic nucleus and pars reticulata of the substantia nigra after 1 week. Expression of immediate early genes (IEGs; c-fos and NGFI-A) was observed in the cortex, thalamic nuclei, subthalamic nucleus, pars reticulata of the substantia nigra, lateral and medial geniculate bodies on the ipsilateral side from 3 to 15 h after FeCl2 injection. Pre-treatment with dimethyl sulfoxide, a hydroxyl radical scavenger, prevented FeCl2-induced expression of IEGs in the thalamic nuclei and geniculate bodies, but not in the cerebral cortex. On the other hand, the effects of FeCl3 were faint and limited on IEGs expression and tissue damage. These results suggest that ferrous iron affects the nervous system vigorously, possibly yielding free radicals such as hydroxyl radicals, and could be one of the important candidates for neurodegenerative diseases under the state in which acclimating systems for iron toxicity are disrupted.     

Differential induction of immediate early genes by excitatory amino acid receptor types in primary cultures of cortical and striatal neurons.

In primary cultures of neurons from cerebral cortex and striatum, 30 s stimulation with the excitatory amino acid glutamate elicited a 5 to 9-fold increase in immediate early gene (IEG) mRNAs. Glutamate increased c-fos, c-jun, jun-B, and NGFI-A (zif/268) mRNAs by binding to both alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor types, and increased c-fos, jun-B, and NGFI-A mRNAs by binding to the metabotropic receptor. NMDA receptor activation elicited IEG expression by a transmembrane calcium influx; AMPA receptor-induced depolarization played a permissive role for the opening of the NMDA receptor channel. The protein kinase C (PKC) inhibitor H-7 (but not inhibitors of cyclic nucleotide-dependent and calcium/calmodulin-dependent protein kinases) partially blocked IEG expression induced by glutamate.     

The effects of repeated induction of long-term potentiation in the dentate gyrus

Previous experiments have shown that long-term potentiation (LTP) generally lasts for only a few days or weeks. The LTP phenomenon would be more attractive as a memory model if it were more enduring. The experiments reported in this paper were designed to test the effects of repeated induction of LTP on the duration of LTP. Three groups of animals received 5 LTP-inducing stimulation sessions. In one group of animals, the stimulation sessions were administered every 24 h. In the 2nd group, the sessions were administered after the LTP effects had decayed to 50% of the peak values. In the 3rd group, the sessions were administered only after the response amplitudes had completely returned to pre-LTP baseline levels. None of the LTP measures were altered, in any group, as a result of repeated induction of LTP. The thresholds, the asymptotic levels of potentiation, and the decay rates were the same after each session. Other treatments may alter the duration of LTP effects, but repeated induction of LTP does not appear to have any lasting effects.     

Plasticity of first-order sensory synapses: interactions between homosynaptic long-term potentiation and heterosynaptically evoked dopaminergic potentiation.

Persistent potentiations of the chemical and electrotonic components of the eighth nerve (NVIII) EPSP recorded in vivo in the goldfish reticulospinal neuron, the Mauthner cell, can be evoked by afferent tetanization or local dendritic application of an endogenous transmitter, dopamine (3-hydroxytyramine). These modifications are attributable to the activation of distinct intracellular kinase cascades. Although dopamine-evoked potentiation (DEP) is mediated by the cAMP-dependent protein kinase (PKA), tetanization most likely activates a Ca2+-dependent protein kinase via an increased intracellular Ca2+ concentration. We present evidence that the eighth nerve tetanus that induces LTP does not act by triggering dopamine release, because it is evoked in the presence of a broad spectrum of dopamine antagonists. To test for interactions between these pathways, we applied the potentiating paradigms sequentially. When dopamine was applied first, tetanization produced additional potentiation of the mixed synaptic response, but when the sequence was reversed, DEP was occluded, indicating that the synapses potentiated by the two procedures belong to the same or overlapping populations. Experiments were conducted to determine interactions between the underlying regulatory mechanisms and the level of their convergence. Inhibiting PKA does not impede tetanus-induced LTP, and chelating postsynaptic Ca2+ with BAPTA does not block DEP, indicating that the initial steps of the induction processes are independent. Pharmacological and voltage-clamp analyses indicate that the two pathways converge on functional AMPA/kainate receptors for the chemically mediated EPSP and gap junctions for the electrotonic component or at intermediaries common to both pathways. A cellular model incorporating these interactions is proposed on the basis of differential modulation of synaptic responses via receptor-protein phosphorylation.     

Diabetes mellitus concomitantly facilitates the induction of long-term depression and inhibits that of long-term potentiation in hippocampus

Memory impairments, which occur regularly across species as a result of ageing, disease (such as diabetes mellitus) and psychological insults, constitute a useful area for investigating the neurobiological basis of learning and memory. Previous studies in rats found that induction of diabetes (with streptozotocin, STZ) impairs long‐term potentiation (LTP) but enhances long‐term depression (LTD) induced by high‐ (HFS) and low‐frequency stimulations (LFS), respectively. Using a pairing protocol under whole‐cell recording conditions to induce synaptic plasticity at Schaffer collateral synapses in hippocampal CA1 slices, we show that LTD and LTP have similar magnitudes in diabetic and age‐matched control rats. But, in diabetic animals, LTD is induced at more polarized and LTP more depolarized membrane potentials (V_ms) compared with controls: diabetes produces a 10 mV leftward shift in the threshold for LTD induction and 10 mV rightward shift in the LTD–LTP crossover point of the voltage–response curve for synaptic plasticity. Prior repeated short‐term potentiations or LTP are known to similarly, though reversibly, lower the threshold for LTD induction and raise that for LTP induction. Thus, diabetes‐ and activity‐dependent modulation of synaptic plasticity (referred to as metaplasticity) display similar phenomenologies. In addition, compared with naïve synapses, prior induction of LTP produces a 10 mV leftward shift in V_ms for inducing subsequent LTD in control but not in diabetic rats. This could indicate that diabetes acts on synaptic plasticity through mechanisms involved in metaplasticity. Persistent facilitation of LTD and inhibition of LTP may contribute to learning and memory impairments associated with diabetes mellitus.